The present invention relates to reactors for the processing of materials in or carried by a gaseous phase by means of corona discharges.
Considerable effort is being expended on the development of techniques for carrying out processes in the gaseous phase, using species which have been activated by corona discharges. Corona discharges occur in gaseous media when the localised electric field in the neighbourhood of a body exceeds the electrical breakdown voltage of the gaseous medium.
Existing corona discharge reactors consist of a chamber having an inlet and an outlet for a gaseous medium, an axial inner electrode and a cylindrical outer electrode surrounding the inner electrode. The electrical discharge within such reactors consists of streamers extending from the inner electrode towards the outer electrode. For the effective processing of a gaseous medium passing through the reactor, it is necessary to produce as many corona streamers as possible because any space which does not contain such streamers is dead space as far as the processing of the gaseous medium is concerned.
In one type of known corona discharge reactor the central electrode is in the form of a wire However, the rapid fall-off in the electric field in a central wire electrode type of corona discharge reactor means that the corona discharge streamers propagate only a short distance from the central wire. This phenomenon limits the efficiency with which gaseous medium passing through the reactor can be treated.
Existing central wire electrode corona discharge reactors have diameters of a few centimeters and lengths of a few tens of centimeters. As a result, to process reasonable volumes of gas, high flow rates are required, which in turn tend to increase the mechanical instability of the central electrode due to aerodynamic effects as well as leading to short residence times in the reactor chamber of the medium to be processed. Hence, existing corona discharge reactors are limited inherently in their effectiveness.
Other corona discharge reactors, see for example, our earlier patent GB 2 282 738, U.S. Pat. Nos. 5,041,145, 5,268,151 or 4,966,666, make use of central electrodes which have a larger diameter. Among other things this reduces the electric field gradient in the region of the central electrode, but there still remains a limitation on the separation between the inner and outer electrodes if an effective corona discharge is to be maintained. Merely increasing the length of corona discharge reactors does not provide an answer to the problem because the gas flow resistance becomes excessive. Also, corona discharge reactors operate in a pulsed manner, and the time taken for an energising pulse to traverse the length of the central electrode, provides another factor which limits the length of a corona discharge reactor.
GB specification 2,008,369A discloses an ozone generator which includes a plurality of parallel electric discharge chambers each of which has a central wire electrode. The wires are supported at their ends by two grid structures to one of which a common feed wire is connected to which, in use a d.c. voltage is applied.
As the device is operated in a d.c. mode, no a.c. current distribution effects have to be considered.
GB patent 1,589,394 discloses an ozone generator which includes a number of parallel corona discharge chambers. A single power supply, which may produce pulsed d.c., a.c., or a mixture of both, potentials is used, but no attempt is made to equalise the distribution of the power supplied to the discharge chambers, either in terms of magnitude or time.
U.S. Pat. No. 4,495,043 discloses an ozoniser in which there is a plurality of ozone producing chambers which are connected to a single pulsed a.c. power supply. However, the ozone producing chambers are not operated simultaneously or, continuously, but are operated sequentially in a pulsed a.c. mode, the duration of each pulse being related to the passage time of a pulse of ozone-producing gas through the corresponding ozone producing chamber, and the intervals between the pulses applied to a given ozone-producing chamber being such that ozone produced by one power pulse is cleared from the chamber before the next power pulse is applied to that ozone-producing chamber.
The patent is concerned mainly with the design of the power supply. The question of the electrical relationship between the ozone-producing chambers is not addressed at all.
U.S. Pat. No. 5,009,858 discloses an ozoniser in which ozone is produced by a silent electric discharge in a number of chambers operated in parallel from a common power supply. The electrical relationship between one ozone producing chamber and another is not discussed at all, but it would appear that the ozone producing. chambers are operated in a continuous a.c. mode.
It is an object of the present invention to provide an improved form of corona discharge reactor for use in the processing of gaseous media by means of electrically activated species.
The term gaseous medium includes the case when one or more constituents of the medium is in the form of an aerosol, or finely divided solid matter carried by a gaseous phase.
According to the present invention there is provided a corona discharge reactor for use in the processing of gaseous media by means of an electrical discharge, including a plurality of individual cylindrical reactor chambers each of which has a central electrode having radial projections thereon, and a concentric outer electrode, a conduit for admitting a gaseous medium to the reactor chambers to be processed in parallel thereby, an outlet for processed gaseous medium from the reactor chambers, and an electrical power supply having power supply lines for applying when switched on a sequence of pulses of electrical energy across the electrodes of the reactor chambers, wherein the electrical impedances of the reactor chamber and the associated power supply lines are matched to one another so that the pulses of electrical energy are applied equally to the individual reactor chambers at predetermined intervals.
Preferably the arrangement is such that the pulse of electrical energy is applied simultaneously to each of the individual reactor chambers.
The central electrode may be solid or hollow so that a cooling medium can be circulated through it. As in the corona discharge reactor described in our earlier patent GB 2 282 273 B the central electrodes can consist of a rod or tube with the radial projections machined upon its surface as a screw thread, or parallel fins, with or without an insulating material filling the spaces between the fins, or they can be made up of a stack of metal disks assembled upon a central conductor either with disks of an insulating material between them, or simply spaced apart with a gas gap between adjacent disks.
The outer electrode can be machined from a solid billet of metal, but preferably it is fabricated as a stack of metal plates which may be separated by other plates of insulating material, or, again, simply held apart, for example by spacers, which may also be of metal. Electrical connection between the metal plates, where these are separated from one another by insulating plates, is made by a number of rods which also serve to locate the components of the outer electrode assembly and clamp them together. This form of construction has the advantage that the edges of the holes in the metal plates can be tapered to form a sharp edge which facilitates the generation of the corona discharge in each of the reactor chambers. Alternatively, the holes in the insulating plates or metal spacers can have a diameter larger than those in the metal plates, so as to provide two sharp edges per plate instead of one.
Preferably the insulating material, where used in the construction of both electrodes, is a temperature resistant material such as a ceramic material.
If it is desired to use corona discharges of the so-called silent type, the passages in the outer electrode can have a cylinder of an insulating, preferably heat-resistant, material inserted in them.
In a preferred arrangement there are six passages through the outer electrode disposed at the apices of a regular hexagon. The advantage of this arrangement is that a single power input can be fed to the centre of the hexagon and thence to the individual reactor chambers via six identical conductors, thus easily satisfying the preferred criterion of applying a pulse of energy to each of the reactor chambers simultaneously. Other numbers of passages, and hence reactor chambers, can be used; the preferred criterion is that the central electrodes of the reactor chambers should be disposed around the circumference of a circle. In such arrangements, fed by a single power supply, it is important that the individual reactor chambers have matching electrical characteristics.
Other arrangements of reactor chambers, for example a square array, can be used, but it is then more difficult to match each reactor chamber to the power supply to ensure that equal pulses of energy are applied simultaneously to each of the reactor chambers.